One possibility: The crack opened as workers created a huge hole in the reactor building's fortress-thick outer wall to remove some old equipment.

"It looks like it's very new," Progress Energy spokeswoman Jessica Lambert said of the crack.

The gap was discovered about six weeks ago, shortly after the nuclear plant was shut down for a major maintenance project, officials said. No radiation escaped.

The crack formed about 9 inches deep inside a containment wall that is 42 inches thick, reinforced with multiple layers of steel bars and tightened steel "tendons," plus lined on the inside with steel plate.

Since then, Progress Energy has worked to assess how far the crack extends along the wall and what repairs are necessary.

It's too soon to know what repairs will cost or whether the plant shutdown, originally scheduled to end in December, will need to be extended, Lambert said.

"We want to ensure before we go back to operating the plant that everything is safe," she said.

It's also too early to say whether repairs could affect the bills of Progress Energy customers. Lambert said any such assessment would have to go through the Public Service Commission.

The Nuclear Regulatory Commission sent a special inspection team to the plant in early October and says it will monitor the repair before the plant goes back into operation.

In a statement, NRC regional administrator Luis Reyes said the crack's discovery "does not appear to represent a major reduction in safety, and there are no immediate concerns because the plant is shut down."

On Friday, the NRC is scheduled to get more details at a public meeting requested by the utility to provide an update on its investigation.

The gap was discovered shortly after the nuclear plant went off-line on Sept. 26 for refueling, maintenance and the replacement of two huge steam generators inside the containment building.

Such shutdowns take place every two years and typically last 30 days. But Lambert said this stoppage was expected to take about three times as long so workers could remove and replace the generators, each of which measures 75 feet long and weighs 550 tons.

To remove the old generators, workers began cutting a hole 23 feet wide and 23 feet high in the wall of the 168-foot-tall building. They found the crack while using an ultra-high-powered water jet to cut through the concrete, she said.

The steam generators had been in the plant since it opened in 1977, Lambert said. Plans to replace them began six years ago. The new units were built in Canada and shipped by barge to Crystal River. They cost $65 million.

The practice of cutting a hole in the wall of the containment building is standard throughout the industry, Lambert said.

Since the discovery, the utility has looked at the rest of the containment building and has not found any other cracks, she said. The old steam generators also have been removed and the new ones installed.

Later, when the hole is closed, workers will reattach and re-tighten the steel tendons, replace the steel reinforcing bars and weld the cut-out steel plate back into place. To have enough concrete on hand for the job, the utility will have its own plant on site.

Progress Energy's preliminary analysis of the crack suggests that it opened recently and might have formed during the operation to open the massive hole in the containment wall, Lambert said.

The utility has raised the possibility that the process of releasing the tension from the steel tendons inside the wall somehow contributed to the crack's formation, NRC spokesman Roger Hannah said.

The containment building houses the plant's nuclear reactor, steam generators and other pumps and valves. (The electrical generators are in another building.) The structure is engineered to contain the rapid buildup of heat and pressure that would take place if a large pipe broke. It has passed previous tests, Hannah said.

The NRC's special inspection is reviewing all circumstances surrounding the crack, considering any effects on the plant's inspection program and determining whether the crack presents issues for other nuclear plants with similar designs. But NRC officials noted that no similar gaps have turned up elsewhere.

Once complete, the inspection team's written report will be made available to the public. The NRC also said it expects to hold a meeting, tentatively scheduled for December or early January, in Crystal River to discuss its team's preliminary findings.

Richard Danielson can be reached at Danielson@sptimes.com or (813) 226-3403.

To LEARn MORE

Crystal River nuclear plant

What: Progress Energy is scheduled to update the Nuclear Regulatory Commission about what it has learned about a half-inch gap found inside the containment wall at its Crystal River nuclear plant.

When: 9 a.m. to noon Friday.

Where: NRC offices in Rockville, Md., but the public can listen in on a conference call. To do that, contact Farideh Saba at (301) 415-1447 or Farideh.Saba@nrc.gov or Trace Orf at (301) 415-2788 or Tracy.Orf@nrc.gov.

to learn more

Crystal River nuclear plant

What: Progress Energy is scheduled to update the Nuclear Regulatory Commission about what it has learned about a half-inch gap found inside the containment wall at its Crystal River nuclear plant.

When: 9 a.m. to noon Friday

Where: NRC offices in Rockville, Md., but the public can listen in on a conference call. To do that, contact Farideh Saba at (301) 415-1447 or Farideh.Saba@nrc.gov or Trace Orf at (301) 415-2788 or Tracy.Orf@nrc.gov.